# Publications

## Discord as a consumable resource

Pacific Rim Conference on Lasers and Electro-Optics, CLEO - Technical Digest (2013)

Quantum discord is conjectured to be a more general quantum resource than entanglement. We support this conjecture by showing, via experimental Gaussian optics, that quantum processors can harness discord to perform tasks classical counterparts cannot. © 2013 IEEE.

## Quantum information: Are we nearly there yet?

New Scientist **219** (2013) 8-

Living systems might be employing quantum information processing. If we can learn their secrets, personal quantum computers no longer seem so fanciful an idea. © 2013 Reed Business Information Ltd, England.

## Requirement of dissonance in assisted optimal state discrimination.

Sci Rep **3** (2013) 2134-

A fundamental problem in quantum information is to explore what kind of quantum correlations is responsible for successful completion of a quantum information procedure. Here we study the roles of entanglement, discord, and dissonance needed for optimal quantum state discrimination when the latter is assisted with an auxiliary system. In such process, we present a more general joint unitary transformation than the existing results. The quantum entanglement between a principal qubit and an ancilla is found to be completely unnecessary, as it can be set to zero in the arbitrary case by adjusting the parameters in the general unitary without affecting the success probability. This result also shows that it is quantum dissonance that plays as a key role in assisted optimal state discrimination and not quantum entanglement. A necessary criterion for the necessity of quantum dissonance based on the linear entropy is also presented. PACS numbers: 03.65.Ta, 03.67.Mn, 42.50.Dv.

## A framework for phase and interference in generalized probabilistic theories

New Journal of Physics **15** (2013)

Phase plays a crucial role in many quantum effects including interference. Here we lay the foundations for the study of phase in probabilistic theories more generally. Phase is normally defined in terms of complex numbers that appear when representing quantum states as complex vectors. Here we give an operational definition whereby phase is instead defined in terms of measurement statistics. Our definition is phrased in terms of the operational framework known as generalized probabilistic theories or the convex framework. The definition makes it possible to ask whether other theories in this framework can also have phase. We apply our definition to investigate phase and interference in several example theories: classical probability theory, a version of Spekkens' toy model, quantum theory and box-world. We find that phase is ubiquitous; any non-classical theory can be said to have non-trivial phase dynamics. © IOP Publishing and Deutsche Physikalische Gesellschaft.

## Operational Significance of Discord Consumption

2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE AND INTERNATIONAL QUANTUM ELECTRONICS CONFERENCE (CLEO EUROPE/IQEC) (2013)

## Topological features of good resources for measurement-based quantum computation

MATHEMATICAL STRUCTURES IN COMPUTER SCIENCE **23** (2013) 441-453

## Quantum Non-Demolition Detection of Polar Molecule Complexes: Dimers, Trimers, Tetramers

Laser Physics: international journal **23** (2013) 015501-

The optical nondestructive method for in situ detection of the bound states of ultracold polar molecules is developed. It promises a minimally destructive measurement scheme up to a physically exciting quantum non-demolition (QND) level. The detection of molecular complexes beyond simple pairs of quantum particles (dimers, known, e.g., from the BEC-BCS theory) is suggested, including three-body (trimers) and four-body (tertramers) complexes trapped by one-dimensional tubes. The intensity of scattered light is sensitive to the molecule number fluctuations beyond the mean-density approximation. Such fluctuations are very different for various complexes, which leads to radically different light scattering. This type of research extends "quantum optics of quantum gases" to the field of ultracold molecules. Merging the quantum optical and ultracold gas problems will advance the experimental efforts towards the study of the light-matter interaction at its ultimate quantum level, where the quantizations of both light and matter are equally important.

## Local characterization of one-dimensional topologically ordered states

PHYSICAL REVIEW B **88** (2013) ARTN 125117

## Correlations in quantum physics

International Journal of Modern Physics B **27** (2013)

We provide a historical perspective of how the notion of correlations has evolved within quantum physics. We begin by reviewing Shannon's information theory and its first application in quantum physics, due to Everett, in explaining the information conveyed during a quantum measurement. This naturally leads us to Lindblad's information theoretic analysis of quantum measurements and his emphasis of the difference between the classical and quantum mutual information. After briefly summarizing the quantification of entanglement using these ideas, we arrive at the concept of quantum discord, which naturally captures the boundary between entanglement and classical correlations. Finally we discuss possible links between discord, which the generation of correlations in thermodynamic transformations of coupled harmonic oscillators. © 2013 World Scientific Publishing Company.

## Extracting quantum work statistics by single qubit interferometry

Optics InfoBase Conference Papers (2013)

We first derive the Jarzynski relation [1] between the average exponential of the thermodynamical work and the exponential of the difference between the initial and final free energy. We then comment on the information-theoretic underpinning of Jarzynski's reasoning which helps explain why the Jarzynski relation holds identically both quantumly and classically [2]. We then present a scheme to verify the quantum non-equilibrium fluctuation relations as encapsulated by Jarzynski. We show that the characteristic function of the work distribution of a quantum system (which is basically equal to the Wick rotated average exponential of the thermodynamical work) can be extracted from Ramsey interferometry of a single probe qubit (which need not itself be pure, though it must not be fully depolarized) [3,4]. An interesting fact is that while the quantum version of the Jarzynski equality remains satisfied even in the presence of quantum correlations, the individual thermodynamical work moments in the expansion of the free energy are, in fact, sensitive to the genuine quantum correlations [5]. Whether this is a fortuitous coincidence remains to be seen, but it certainly goes towards explaining why the laws of thermodynamics happen to be so robust as to be independent of the underlying micro-physics. We intend to elucidate the subtle connection between Jarzynski's relation and the "quantum arrow of time". Finally we comment on the fact that our scheme for measuring the quantum work characteristic function belongs to the computational class known as DCQ1 [6], namely all computations that can be performed with only one pure qubit (and N maximally mixed ones). © OSA 2013.

## Extracting quantum work statistics by single qubit interferometry

Optics InfoBase Conference Papers (2013)

We first derive the Jarzynski relation [1] between the average exponential of the thermodynamical work and the exponential of the difference between the initial and final free energy. We then comment on the information-theoretic underpinning of Jarzynski's reasoning which helps explain why the Jarzynski relation holds identically both quantumly and classically [2]. We then present a scheme to verify the quantum non-equilibrium fluctuation relations as encapsulated by Jarzynski. We show that the characteristic function of the work distribution of a quantum system (which is basically equal to the Wick rotated average exponential of the thermodynamical work) can be extracted from Ramsey interferometry of a single probe qubit (which need not itself be pure, though it must not be fully depolarized) [3,4]. An interesting fact is that while the quantum version of the Jarzynski equality remains satisfied even in the presence of quantum correlations, the individual thermodynamical work moments in the expansion of the free energy are, in fact, sensitive to the genuine quantum correlations [5]. Whether this is a fortuitous coincidence remains to be seen, but it certainly goes towards explaining why the laws of thermodynamics happen to be so robust as to be independent of the underlying micro-physics. We intend to elucidate the subtle connection between Jarzynski's relation and the "quantum arrow of time". Finally we comment on the fact that our scheme for measuring the quantum work characteristic function belongs to the computational class known as DCQ1 [6], namely all computations that can be performed with only one pure qubit (and N maximally mixed ones). © OSA 2013.

## Nonclassicality of optomechanical devices in experimentally realistic operating regimes

Physical Review A - Atomic, Molecular, and Optical Physics **88** (2013)

Enforcing a nonclassical behavior in mesoscopic systems is important for the study of the boundaries between the quantum and the classical world. Recent experiments have shown that optomechanical devices are promising candidates to pursue such investigations. Here we consider two different setups where the indirect coupling between a three-level atom and the movable mirrors of a cavity is achieved. The resulting dynamics is able to conditionally prepare a nonclassical state of the mirrors by means of projective measurements operated over a pure state of the atomic system. The nonclassical features are persistent against incoherent thermal preparation of the mechanical systems and their dissipative dynamics. © 2013 American Physical Society.

## Wigner rotations and an apparent paradox in relativistic quantum information

PHYSICAL REVIEW A **87** (2013) ARTN 042102

## Majorana fermions in s-wave noncentrosymmetric superconductor with Dresselhaus (110) spin-orbit coupling

Physical Review B - Condensed Matter and Materials Physics **87** (2013)

The asymmetric spin-orbit interactions play a crucial role in realizing topological phases in a noncentrosymmetric superconductor (NCS). We investigate the edge states and the vortex core states in the s-wave NCS with Dresselhaus (110) spin-orbit coupling by both numerical and analytical methods. In particular, we demonstrate that there exists a semimetal phase characterized by the flat Andreev bound states in the phase diagram of the s-wave Dresselhaus NCS which supports the emergence of Majorana fermions. The flat dispersion implies a peak in the density of states which has a clear experimental signature in the tunneling conductance measurements and the Majorana fermions proposed here should be experimentally detectable. © 2013 American Physical Society.

## Comment on "quantum szilard engine"

Physical Review Letters **111** (2013)

A Comment on the Letter by S. W. Kim, Phys. Rev. Lett. 106, 070401 (2011).PRLTAO0031-900710.1103/PhysRevLett.106.070401 The authors of the Letter offer a Reply. © 2013 American Physical Society.

## Extracting quantum work statistics and fluctuation theorems by single-qubit interferometry

Physical Review Letters **110** (2013)

We propose an experimental scheme to verify the quantum nonequilibrium fluctuation relations using current technology. Specifically, we show that the characteristic function of the work distribution for a nonequilibrium quench of a general quantum system can be extracted by Ramsey interferometry of a single probe qubit. Our scheme paves the way for the full characterization of nonequilibrium processes in a variety of quantum systems, ranging from single particles to many-body atomic systems and spin chains. We demonstrate our idea using a time-dependent quench of the motional state of a trapped ion, where the internal pseudospin provides a convenient probe qubit. © 2013 American Physical Society.

## Witnessing the quantumness of a single system: From anticommutators to interference and discord

Physical Review A - Atomic, Molecular, and Optical Physics **87** (2013)

We introduce a method to witness the quantumness of a system. The method relies on the fact that the anticommutator of two classical states is always positive. By contrast, we show that there is always a nonpositive anticommutator due to any two quantum states. We notice that interference depends on the trace of the anticommutator of two states, and it is therefore operationally more suitable to detect quantumness by looking at anticommutators of states rather than their commutators. © 2013 American Physical Society.

## Information-theoretic lower bound on energy cost of stochastic computation

PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES **468** (2012) 4058-4066